Brake systems of modern motor vehicles generally include a number of safety functions or auxiliary devices such as an anti-lock system (ABS) or an electronic stability program (ESP), which are meant to contribute to enhancing the safety of the motor vehicle especially during braking. Typically, an electronic control unit (ECU) is employed for the control of these systems. Most various parameters such as tire condition variables in particular are required as input signals for this control unit. Predominantly, the tire condition variables shall be used for the automated increase of vehicle safety by electronically controlled brake reactions. In addition, there is the possibility of indicating to the driver interesting information such as the measured tire pressure, by way of showing it on separate display units.
Among others, physical conditions are determined as tire condition variables such as the inside air pressure, air temperature, temperature of the tire rubber, rubber deformations, or noise spectra of the tire, but also characteristic values or characteristics of the tire, from which age, structural shape or the type of the tire, such as summer or winter tire, can be taken. Further, e.g. wheel parameters are measured that are required for driving dynamics control of the motor vehicle such as lateral tire force, wheel torque, or the tire contact force.
Tire condition variables are usually determined or measured on one wheel, on several or all wheels of the motor vehicle. An appropriate transmission system is required in order to transmit the signals measured by the rotating wheel or tire to the motor vehicle. In prior art transmission systems, wherein tire condition variables are transmitted wirelessly to a receiver in the motor vehicle by way of an active sensor in the wheel, it is disadvantageous that, given usual transmitting field intensities, the received signal can be received in every receiving device, unfavorably even in a neighboring vehicle, because transmitters generally use a like carrier frequency range. When developing a transmission device for tire condition variables, there is the problem in the application of high-frequency methods that one objective is to achieve a sufficient radius of the service area, while on the other hand another objective is to appropriately suppress common channel interferences or to avoid other undesirable couplings between several transmitters and receiving devices in addition.
To avoid these problems, a so-called wheel-hub coupler is known in the art, which allows transmitting data between the rotating wheel and the motor vehicle in a wireless fashion. The function principle herein is an inductive coupling of wheel and motor vehicle. To this end, a coupling device or a wheel-hub coupler is arranged in one or more wheel hubs of the motor vehicle, and the coupling device for generating a field coupling is composed of a rotor and a non-rotating stator. The one or more field coupling elements are opposed to each other, separated by a narrow air slot, and coupling takes place through electric fields and/or magnetic fields, and/or electromagnetic fields. The advantage of this wheel-hub coupler renders it possible to transmit not only signals of measuring systems, but also energy into the interior of a tire for the supply of a measuring system. Further, a dynamo can be integrated into the wheel-hub coupler to supply the measuring systems arranged in the tire interior.
The coupling device made up of stator and rotor substantially includes a number of stator and rotor windings for an inductive coupling, and these windings can be designed as flat coils and cylindrical coils, while the windings of stator and rotor are spaced from each other by a narrow air slot. Depending on the desired transferable power and/or the desired requirements with regard to a shielding, stator and rotor members in the winding area can be designed as ferrite or sheet iron half shells or shielding half shells. These are mounted into the windings in such a fashion that they embrace like shells the windings of rotor and stator jointly in the type of a mantle core with the air slot.
A shortcoming of the wheel-hub coupler described above is that a wheel rotational speed sensor is required at each wheel hub in addition with the wheel-hub coupler for use in a motor vehicle, which is equipped with both a system of measurement with wheel-hub coupler and an ABS brake system. It is problematic that wheel-hub coupler and wheel rotational speed sensor, depending on their construction, compete with each other for the same mounting space. Besides, the magnetic field of the wheel-hub coupler can cause interferences at the rotational speed sensor, because the mode of operation thereof can also found on measuring magnetic fields.
Another disadvantage in the use of wheel rotational speed sensors, which usually found on Hall probes or magnetoresistive sensors, wherein the signal-generating structure such as an encoder, grid or toothed wheel is scanned in dots, is the danger of a so-called pole pitch error appearing due to process tolerances of the signal-generating structure. The pole pitch error implies that the sectors cover different angles so that with a constant rotational speed, variation of the instantaneous rotational speed may be faked, what can cause malfunctions in control systems such as ABS.
In view of the above, an object of the invention is to provide a device for transmitting signals about tire condition variables from a wheel to an electronic controlling or evaluating device arranged in the area of the vehicle body, wherein the tire condition variables can be sensed especially by one or more sensor subassemblies, which are arranged in a wheel rotatably mounted above a wheel hub on the chassis, and the purpose of the transmission device is to determine the rotational speed of the wheel.